this PDF file - Mah-Jong - British Rules

Knitting and Triple Knitting
More times than you would wish for you are dealt a mixture of tiles from two or
three suits, and virtually no honour or major tiles.
One’s first reaction tends to be despair and to think that maybe next time it will
be better.
However, there is some hope. Because you could consider going for the special
hand of Knitting. Or maybe Double Knitting. Both score 500 points.
Knitting is when you get 7 pairs of the same numbers in any 2 suits.
Double Knitting is when you get 4 sets of the same numbers in the 3 suits, plus a
pair with the same numbers in any 2 suits.
As with a number of the special hands you are hampered because you cannot
claim a pung or kong along the way. And also your inevitable discards of dragons
and winds makes it evident that you are collecting one of these particular special
hands.
The upside is that other players have little idea of what particular tiles you are
looking for.